One of the problems with conventional ailerons is that they provide more than adequate roll control at high speed and not enough at low speed. Thus, the configuration of the aileron is often a compromise where it is oversized for high speed flight and undersized for low speed operation. One prior art method to alleviate this problem is to have high speed midspan ailerons and wing tip ailerons for low speed operation. Additionally, spoilers can be used to augment control. However, the spoiler system does not provide a pure rolling moment but results in an overall loss of lift. Another approach to alleviating this problem is to use trailing edge flaps to augment the otherwise conventional wing tip aileron. However, this requires high rate flap actuators which in turn have penalties in terms of weight and cost.
Airfoils typically used for new transport planes have a fairly high amount of camber on the trailing edge. This results in a cusp on the lower surface and a fairly high boattail angle on the upper surface. The aileron is generally located in this aft portion of the airfoil. The adverse pressure gradients associated with the high boattail angles on the upper surface create a flow condition which causes separation at increased angles of attack or any further increase in boattail angle. Thus, downward deflections more than a moderate amount (e.g. 10.degree. down) do not produce any appreciable increase in lift, since the flow on the upper surface becomes separated. This means that roll control for higher deflection angles in a downward direction becomes very non-linear and ineffective. This is not a problem for high speed flight, because in general only small aileron deflections are required, and the resulting loss in lift is inconsequencial. However, a lift loss at low altitude and low speed, such as during final approach, or during take-off, is highly undesirable. Also, it should be noted that a military transport plane, and particularly an airplane with STOL capabilities, has to perform more severe roll maneuvers than a commercial transport airplane. The slower speeds in a STOL mission make the airplane more sensitive to gust and cross-wind. Also, a military transport, particularly in a STOL mission, will have to fly into fields with poor or no approach guidance system. This means that the airplane has to be able to make major course corrections close to the ground, when landing in poor weather conditions where there is reduced visibility. A roll control system that produces a net loss in airplane lift is therefore dangerous to this application.
In view of the foregoing, it is an object of the present invention to provide an aileron system and method of using the same for effective operation at both high and low speeds, without significant loss of lift in performing a roll maneuver in low speed operation.